Duncker's father was a German who emigrated to the Transvaal in South Africa in 1880. There he married a Boer woman and had two sons and one daughter. After 10 years in Transvaal he lost his German civil rights and became a citizen of Transvaal, as did his children. After the Boer War (1899-1902), in which he and his sons were involved, they returned to Germany in 1903 and again were granted German civil rights. Therefore Duncker had German nationality when he competed at the Olympics in 1906, which has been disputed. Duncker won multiple German championships, as follows: 1905 110 m hurdles, 1906-07 100 and 400 metres and the 110 metre hurdles; and was British AAA Champion at the 120 yard hurdles in 1908. Prior to IAAF ratification of world records, he was credited with world bests of 10.8 for 100 metres in 1905, and 57.2 for the 440 yard hurdles in 1909. After Duncker finished his studies in 1907 he returned to South Africa and was entered for the 1908 Olympics Games for South Africa, despite still being a German, but he was unable to start due to a severe attack of rheumatism.
Personal Bests: 50 – 5.4 (1905); 100 – 10.8 (1905); 400 – 53.0 (1905); 800 – 2:13.2 (1907); 110H – 15.2y (1909); LJ – 6.06 (1906); DT – 25.70 (1905).